Update mutable methods to be final and use assertMutable. Refactor heirarchy of handlers that don't need MessageFunctional. Add newInstance to MessageFunctional so that handlers can be mutated by the operation handle. Add convenience constructors and builders to some handlers. Add additional unit tests.
Update mutable methods to be final and use assertMutable. Refactor heirarchy of handlers that don't need MessageFunctional. Add newInstance to MessageFunctional so that handlers can be mutated by the operation handle. Add convenience constructors and builders to some handlers. Add additional unit tests.